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Using its web as a gill, the diving-bell spider can live underwater with only occasional visits to the surface.

The arachnid (Argyroneta aquatica) breathes air from a bubble that it grabs from the surface of water using fine hairs on its abdomen. The spider traps the air within a bell-shaped silken web that it constructs underwater and carries around like an aqualung.

It was first described over 250 years ago, but until now biologists didn't know how it managed to remain underwater without frequent visits to the surface to renew the oxygen in its tiny air bubble.

To find out, Roger Seymour from the University of Adelaide in South Australia and Stefan Hetz of Humboldt University in Berlin, Germany, placed 12 spiders in individual aquaria and measured the oxygen levels within the air bubbles using optical fibres tipped with oxygen-sensitive dye. Seymour also measured the concentration of oxygen in the water outside the bubble.

"The spider's web acts like a gill," says Seymour. The silken web allows oxygen to diffuse from the surrounding water into the depleted air, as well as release carbon dioxide.

Based on the rate that oxygen diffused into the air bubble, Seymour calculated that 70 per cent of the spider's oxygen supply comes from diffusion through the web.

During the day, Seymour never observed the spiders replenishing their air supply. He believes they can survive underwater for more than 24 hours, allowing them to stay out of sight of predators and prey alike. However, as oxygen levels go down, nitrogen concentrations increase in the bubble. Nitrogen starts to diffuse out into the water, and eventually the bubble collapses.

Nature is amazing!

Maybe for once, someone will call me "Sir" without adding, "You're making a scene."